Optimizing the thermophysical qualities of innovative clay–rGO composite bricks for sustainable applications

This work concerned the development of a unique reduced graphene oxide (rGO) nano-filler to provide innovative opportunities in enhancing the thermophysical performance of clay composite bricks. Whereas, a series of clay–rGO composite bricks were produced, doped with various levels of rGO nanosheets (i.e., 0, 1, 2, 4, and 6 wt% clay). Each clay–rGO composite’s microstructure, shrinkage, morphology, density, porosity, and thermophysical characteristics were carefully investigated, and the thermal conductivity performance was optimized. Incorporation of different levels of rGO NPs to the clay matrix allowed all the peaks intensity to rise relative to the untreated one in the XRD pattern. Meanwhile, the inclusion of these doping resulted in a grew in the crystallite sizes and apparent porosity within the compositions. In this vein, shrinkage fracture of fabricated brick composites varied depending on dopants type and levels during the drying and firing processes. Moreover, there are some changes in chemical compositions, as well as wave shifts, suggesting that functional groups of rGO may have contributed to partially introduce carbonyl groups in clay–rGO composites. Besides, the porous topography and bulk density improved rapidly with respect to the plane of the rGO nanosheets within the composites. The differ-dense microstructure displayed in the SEM micrographs supports these outcomes. Remarkably, clay–(4%)rGO compound not only has an optimum thermal conductivity value (0.43 W/mK), but it also has a high heat capacity (1.94 MJ/m3K). These results revealed the exceptional features of rGO sheets such as large surface area with high porosity within the modified clay composites.


Experimental Materials and analytical methods
Precursors for the synthesis of rGO nanosheets included sulfuric acid (H 2 SO 4 ), phosphoric acid (H 3 PO 4 ), graphite powder, potassium permanganate (KMnO 4 , 99%), hydrochloric acid (HCL; 37%) and hydrogen peroxide (H 2 O 2 ; 30%) were used as intermediates.On the other side, clay minerals were gathered in fine grains from the Kharga Oasis, where the clay is formed up of different crystalline mineral compositions.The Kharga Oasis, situated in the Egyptian Western Desert (Lat.25.46-25.78N and Long.30.54-30.91 E), is one of the main depressions.The oasis is bounded on the north and east by a limestone ridge, the precipitous rocks of which form a sharp barrier to the valley floor 37 .However, as you move south and west, the depression's bottom slowly blends with the Nubian Sandstone desert 38 .The Nubian Sandstone series primarily makes up the Kharga Oasis's base and subsurface succession 39 .

Powder synthesis of rGO NPs
A modified hummers method was used to create highly qualified graphene oxide (GO) nanosheets.In addition, the level of functional groups was reduced using an in-situ oxygen plasma surface modification technique to create rGO nanosheets, as described in details in our previous work 40 .

Synthesize of clay-rGO nanocomposite bricks
A series of clay-rGO composite bricks were produced, doped with various levels of rGO nanosheets (i.e., 0, 1, 2, 4, and 6 wt% clay).Figure 1 displays a sequence of clay-rGO composites molding and firing processes, as well as a rGO sheets dispersion diagram within the clay internal structure.Whereas, each hybrid block is composed of up of 50 g of solid components mixed with 40 ml of tempering water.The clay compound's test portions were hand-shaped in a hardwood frame laboratory molding (4.5 cm × 4.5 cm × 2cm).These engineered hybrid materials performed well during molding and dried safely with no obvious cracks.To ensure there was no moisture present, the newly constructed raw bricks were naturally dried at RT for three days before being fired for 4 h at 1100 °C.Due to the abundance of alkali oxides inside the burned samples in this situation, burning at 1100 °C assures higher vitrification, giving the bricks more resistant to water penetration and therefore fewer prone to humidity associated weathering deterioration 20 .

Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.

Characterization of the composite bricks
X-ray Diffractometry (XRD) with a Cu target (Model D8 is an ADVANCE, Bruker) was used to evaluate the mineralogical composition characteristics of the unmodified and modified clay composites.Patterns were seen between 4 and 70° 2θ.Further, Fourier transform infrared (FTIR) spectroscopy (Jasco Model 4100_Japan) was utilized to examine the structural changes caused by chemical modifications of clay substances.At RT, the FTIR measurements yields observations with a resolution of 4 cm −1 in an area of 4000-400 cm −1 .Meanwhile, using a high-resolution scanning electron microscopy (SEM, JEOL JEM-2100) (Japan), the surface morphology of each of Figure 1.Clay-rGO composite bricks during the molding and firing process at at 1100 °C.
the clay-rGO composites was evaluated.Drying and firing shrinkage behaviour were evaluated by estimating the variation in dimensions of the clay brick after drying and firing, respectively, at room temperature.A SETARAM LABSYS Evo thermogravimetric analyzer (TGA) 1600 was employed for the thermal stability investigation, which was conducted in the 23-1000 °C temperature range.In the air surroundings, both cooling and heating rates were 10 °C/min with an isothermal accuracy of ± 1 °C.Thermal characteristics were also recorded with a hot disc thermal constants equipment (heated Disc TPS 2500 S, Göteborg, Sweden).Whereas, this Transient plane source (TPS) method is the most accurate for investigating thermal transport characteristics.Moreover, physical observations of bulk density as well as apparent porosity were performed via the Archimedes technique.

XRD analysis
The XRD technique was employed to investigate the structural changes in the clay structure caused by impregnation with several concentrations of rGO nanosheets (i.e., 0, 1, 2, 4, and 6 wt% clay).Figure 2a,b depict XRD profile peaks of as-synthesized clay-rGO composites in the (4°-80°) and (21°-22°) ranges, respectively.The pristine clay composition contains a variety of mineral types and amounts, which are summarized in Table 1.Consequently, unmodified compound exhibits distinct reflections at 2θ values of 29.4°, 43.3°and 48.5°, which correspond well to calcite (COD card No: 1010928).Illite (COD card No. 9013732) and quartz (COD card No. 1011176) both had diffraction peaks at (2θ = 50.3°,60.2°), and (2θ = 26.5°),respectively.Additionally, kaolinite (COD card No. 9009230) was found at 2θ of the following locations: 12.2°, 20.2°, 24.8°, and 39.4°, as well as the phyllosilicate mineral (2θ = 12.5°) 41 .Meanwhile, 2θ = 21.7°,23.6°, and 27.7° were detected for feldspar at 41,42 .Thereafter, incorporating different levels of rGO NPs to the clay matrix allowed all the peaks intensity to rise relative to the untreated one.Meanwhile, the inclusion of these doping resulted in a newly induced peak at 21.5° (JCPDS card No: 752078), and its intensity grew as more rGO load within the clay-rGO lattice structure.This varying behaviour is clearly depicted in Fig. 2b, which use a zoomed image of peak at 21.5°, respectively.This trend confirms that rGO was effectively combined into the clay support, as well as causing structural defects and disorders in the clay's crystal structure 43 .Similar findings from previous studies were reported 43,44 .
However, the level of rGO sheets employed had an effect on the calculated crystallite size of clay-rGO compounds.Where, using Scherrer's formula, the combination's crystallite size was estimated 45 : (1)   where λ is the wavelength, β is the full width at half maximum of the peak, and θ is the diffraction angle.The crystallite size of the pristine substance was 23.7 nm and gradually grew to 24.3, 27.5, and 35.4 nm with respect to the rGO level up to 1%, 2%, and 4%, respectively, before reducing to 34.2 nm at 6%-rGO.This can be clarified via the reactions that generated after the dopant was introduced, which supports the substantial physiochemical variations of the clay on the rGO surfaces.

FTIR spectra
FTIR spectra were used to evaluate the effect of rGO concentration on the chemical compositions of clay-rGO composites, as seen in Fig. 3.A wave at 3854-3354 cm −1 associated with the H-O expansion vibration of H-bonded adsorbed water molecules in pure clay was detected.Then it progressively vanished in the composites due to the non-polar property of rGO, which reduced the water absorption 27,46,47 .The short band at 2922 cm −1 is associated with C-H stretching mode and indicates that some organic contribution is present 48 .In addition, the signals at 2184 cm −1 and 2020 cm −1 are assigned to C≡H stretching and H 2 O, respectively.Meanwhile, the small signals recorded at 1725 and 1625 cm −1 may be attributed to the C=O and C-H-O, respectively 49 .Besides, carbonate species, Si-O out-of-plane stretching, and amorphous meta kaolinite structural bonds were situated at 1416, 991, and 872 cm −1 , respectively 50,51 .Similar findings have been reported in previous studies 47,52 .Moreover, there are some changes in peak intensities and wave shifts with respect to the plane of the rGO nanosheets within the composites.Whereas, modified samples revealed a slight shift in absorption peaks (3354, 2922, 141, 991, 872, 797, 776 cm −1 ) towards higher wavenumber values as compared to the unmodified sample.The peaks at 797 and 776 cm −1 were attributed to the symmetrical and asymmetrical stretching vibrations of the quartz Si-O-Si bonds between the tetrahedron 27 .The shift in the tetrahedral arrangement, combined by a decrease in the degrees of crystallinity in the tetrahedral plate, may be responsible for the deviation of the Si-O-Si bonds and the transition to higher frequencies 53 .However, the peaks at 2184 cm −1 and 2020 cm −1 , had a movement towards low wavenumbers, indicating the establishment of hydrogen-bonding interaction between clay and rGO NPs 54 .Along the same lines, the observed peak intensity of the composite vibration increased relative to the untreated sample and increased progressively as the compound's rGO level rose.Wherein, the strength of the carbonyl peak at 1416 cm −1 rose as the amount of rGO increased, suggesting that functional groups of rGO may have contributed to partially introduce carbonyl groups in clay-rGO composites 49 .These experiments revealed that rGO and clay may be combined successfully.

Morphology
Figure 4 depicts SEM micrographs and pore size distributions of pristine and treated clay-rGO composites with various rGO additions.Depending on the doping level, the porous topography of the clay-rGO composites can be observed.Whereas, Fig. 4a presents the surface features of the pristine fine-pored clay.Besides, comparable structures are detected in Fig. 4b-e in clay and rGO nanosheets which were incorporated differently.Meanwhile, several wrinkled and crumpled structures were observed in the clay composites.This property of clays is crucial in preventing undesired restacking and aggregation of individual nanosheets 49 .Furthermore, the findings indicate that the rGO functional groups and the clay matrix adhered well to each other, resulting in hydrogen bonds formation and uniform distribution of rGO inside the clay structure 55 .The structure of the composites also expanded at higher rGO levels, revealing an amorphous structure with more micropores, multiple bonds, and increased internal energy 56 .In addition, the average pore sizes for the pure and modified clay-rGO composites Moreover, increasing the quantity of nanosheets in the composites resulted in the opposite behaviour, with the estimated value decreasing to 3.6 µm in the clay-(6%)rGO composite.These observations might be explained by the creation of new intermolecular covalent bonds and hydrogen bonds between clay-H 2 O and rGO functional groups 14 .These bonds influence the rate of hydration crystal growth and lead to the uniform dispersion of nanosheets inside clay-rGO composites 57 .The result was a notable improvement in the surface roughness and thermal characteristics of these composites as a result of the miscibility of nanosheets.
In light of the importance of porosity, which is in alignment with this, the Archimedes technique was employed for calculating the apparent porosity and density characteristics of the created compounds (See Fig. 5).Since, they are one of the most important factors influencing thermal conductivity 2,58 .Pristine clay exhibited the lowest apparent porosity value (24.01%), that subsequently increased to 32.67, 41.81, and 49.89% with regard to the rGO load levels (1, 2, and 4%), respectively.Furthermore, adding more dopant nanosheets resulted in a reversal of the trend, with apparent porosity falling to 42.72% at the 6%-rGO sample.The variation in this pattern is closely correlated with the average pore size values derived from SEM micrographs and depicted in Fig. 5a.4%-rGO nanosheets are clearly the most effective level to optimize the apparent porosity necessary for a good thermal conductor.Since during firing at 1100 °C, bricks with additives have a more porosity nature than ordinary bricks.This is due to the degradation of CaCO 3 which leads to combustion of additives or pores.Meanwhile, as seen in Fig. 5b, the bulk density of the pristine substance was 2.02 g/cm 3 , then reduced to 1.18 g/cm 3 with regard to the rGO level up to 4% before growing to 1.38 g/cm 3 at 6%-rGO.The differ-dense microstructure displayed in the SEM images supports these outcomes.Moreover, the variations in the density trend are driven by the growth of finer particles, which in turn promote densification by raising the rate of atomic diffusion, leading to denser microstructures 59 .Meanwhile, the density of bricks containing additives reduces as a consequence of the chemicals' propensity to decay during the combustion cycle 23,60 .An increase in porosity and a decrease in bulk density can have several of beneficial consequences on the performance as well as the features of burnt clay bricks.The benefits are as follows: lightweight bricks, moisture resistance, reduced shrinkage and cracking, lower thermal expansion, cost savings, energy-efficient burning, sustainability, sound and thermal insulation in buildings.Whereas, adding additional air pockets within the brick regulates temperature and reduces energy consumption, hence improving structural insulation.Before that, studies using various dopants, such as fly ash and TiO 2 NPs, revealed a similar pattern of bulk density behavior 29,[61][62][63] .

Shrinkage behaviour
As demonstrated in Fig. 6, the shrinkage behaviour of the produced brick composites with regard to rGO levels varied during the drying and firing stages.In the first process, the orientation of the chips in soft mud bricks, that are formed by throwing clay into molds, is more random throughout the drying stage, and as a result, their behaviour is more isotropic.Once the formed brick dries, the volume reduces (by at least 6%) and some initial strength develops.As a result, the interior of the brick shrinks slower than the outside.Moreover, shrinkage cracks might appear if water evaporates too rapidly, reducing strength significantly 64,65 .Our literature states that high-quality bricks should shrink 10% when dried.Whereas, the samples exhibit acceptable drying shrinkage with undetected shrinkage fractures.Herein, the dry shrinkage systematically increased from 13.7% of the pristine compound to 16.1% with the addition of 4%-rGO, respectively.The existence of carbonyl functional groups and hydroxyl groups in rGO may be responsible for this outcome because they interacted with clay structure through ionic exchange, hydrogen bonding, and chemical association 28 .
On the other hand, the calcination process, which involves heating composite bricks to a high temperature, causes some minerals in the clay matrix to melt, changing the volume and strength of the brick.Further, as the volumetric mass increases, the brick becomes stronger while maintaining its weight.According to the fact that quality bricks have a firing shrinkage of less than 8%, all of the molds in this study had good firing shrinkage.
At the firing stage in this work, the shrinkage attained was 6.3% of the pristine combination and gradually decreased to an optimal value of 5.2% with the addition of 4%-rGO nanosheets.These enhancement might be ascribed to the hydrophobicity of graphene platelets and the nanofiller-matrix adhesion/interlocking caused by their wrinkled surface, which have better two-dimensional geometry 33,66,67 .Besides, these findings are in line with previous studies that suggested 1 wt% of rGO might enhance the toughness of clay-rGO composites.As, the sheets are connected, some of them overlap, forming strong plates that are firmly attached to the matrix 68 .Additionally, the removal of certain hydroxyl, functional groups, chemically mixed water, and the reduction of substances into ashes, in addition to other activities that clearly lower its volume, may also be directly contributing to the low values of firing shrinkage.Finally, these chemical changes and the repositioning of particles in the crystal lattice that occur during firing result in a more compact solid structure 65,69,70 .

Thermogravimetric Analysis (TGA)
TGA and DTG techniques were performed to investigate the thermal stability of the untreated and treated clay composites depending on the functionalization degree of rGO sheets, as seen in Fig. 7.The dehydration of clay minerals was caused by water molecules that were absorbed into the clay combinations, as seen in Fig. 7a,b.This included both physically received water and bound water associated with interlayer cations in the 25-300 °C range 71 .This resulted in a 0.431 mg and 0.312 mg drop in physical weight for the pristine and modified clay materials, respectively.The shift occurred due to the existence of rGO inside clay layers, thus leading to the inclusion of extra functional groups (including carbonyls (C=O) and carbon atoms SP 2 ).In-situ XRD patterns provide practical support for this findings 72 .Besides, the pure substance recorded a small intensity of the DTG peak at 186 °C, however the clay-rGO instance exhibits a wide and high intensity of the peak.The evolution can be attributed to the presence of oxygen-containing compounds produced by rGO sheets, including carboxylates and epoxy, whose pyrolyzed releasing CO, CO 2 , as well as H 2 O vapour 73,74 .Likewise, the dehydroxylation of kaolinite is the main reason for the weight losses for both treated and untreated composites between 300 and 750 °C75 .In turn, clay-rGO dropped 0.372 mg while pure clay lost 0.489 mg.It is plausible that additional clay minerals, such smectite and illite/mica, also undergo dehydroxylation within this temperature range and very marginally influence the weight reduction.A peak observed at 717 °C and 686 °C on both DTG curves is ascribed to the decomposition of calcite and the dehydroxylation of illite 76 .As for the clay-rGO composite, its DTG curve displayed an apparent peak at 883 °C with 2.621 mg weight loss, suggesting the most stable groups of nanofiller sheets-like carbonyl and quinone-were breaking down.The 10 °C/min ramp rate utilized in this investigation yielded results comparable to mullite or γ-alumina nucleation acceleration.The residual weights of the pristine and modified clay substances during the 25-1000°C temperature range were 96.77% and 93.91%, respectively.This suggests that clay bricks containing rGO have a slightly greater weight loss.Therefore, the high thermal stability of these fired clay bricks leads to great construction durability, thermal insulation, thermal shock resistance, and energy efficiency.

Thermal Properties
The thermal features of clay bricks were significantly influenced by the level of rGO nanosheets incorporated.Table 2 addresses these modifications to the thermophysical performance, including thermal conductivity, thermal diffusion, thermal effusivity, and specific heat capacity.In particular, the thermal conductivity of the pristine clay was 0.32 W/mK, then gradually grew to 0.33, 0.35, and 0.43 W/mK as the rGO level rose to 1, 2, and 4%, respectively, before dropping to 1.38 g/cm 3 at 6%-rGO.These thermal characteristic variations are prompted by the significant interaction between clay-H 2 O products and oxygen functional groups (epoxides, carboxyl's, and hydroxyls) of rGO nanosheets 7,14,77 .Therefore, these interactions influence the rate of hydration crystal growth and result in a uniform dispersion of nanosheets inside the clay-rGO compound 57 .Moreover, to enhance heat conduction, significant thermal diffusivity values are also necessary.Therefore, a material with a high thermal diffusivity has physical relevance as a result of the rapid rate at which its temperature changes when heated 78 .Several previous studies revealed a similar pattern as it came to enhancing the thermal performance of fired clay bricks 7,18,20,21 .The inclusion of a variety of rGO nanosheets within a clay matrix raised both thermal diffusivity and thermal effusivity behaviour.Whereas, clay-(4%)rGO compound not only has an optimum thermal diffusivity value (0.46 mm 2 /S), but it also has a high thermal effusivity (863.88Ws 1 ´2/m 2 K).The relatively high thermal diffusivity of these samples enables heat to spread further and to be absorbed by the substance less 79,80 .
Additionally, it is remarkable to note that the heat capacity of the clay-rGO composites rose (1.15-1.94MJ/ m 3 K) as the GO amount (0-4%) grew, then reduced to 1.5 MJ/m 3 K with the addition of 6%-rGO.The main explanation for these findings is the existence of an appropriate level 4%-rGO sheets, which revealed exceptional features like large surface area with high porosity inside the modified clay composites.Whereas, at lower levels of rGO (1-4%), ion exchange arises with alkaline clay pore solutions of charged ions such as Ca 2+ , Mg 2+ , Na + , and OH − , and the distinctive surface area of the rGO sheets efficiently permitting liquid absorption of the interlayers 57 .Nevertheless, raising the level of 6%-rGO in clay formation may not promote their efficacy since excess sheets lead to enhance rGO agglomeration 81 .Aggregation enabled rGO to lose its intrinsic benefit of having a large specific surface area, thus it is unable to act as a site of nucleation which accelerates the hydration and porosity distribution of clay composites, and affects the thermophysical performance of clay matrix 82 .As a consequence, rGO agglomeration with small levels and an increased specific surface area is selected since it maintains its efficacy inside the structure and improves the performance of the clay compound.According to 83 , all of the specimens meet the required specifications since their thermal conductivity is below 0.6 W/mK.

Conclusion
In this context, the internal clay structure was modified by impregnation with several concentrations of rGO nanosheets (i.e., 0, 1, 2, 4, and 6 wt% clay).The inclusion of rGO in clay matrix at varying proportion significantly affected the growth rate of hydration crystals that turns on the thermophysical properties of composites.Whereas, the compositions' crystallite sizes, bulk densities, and porosity characteristics all improved as a result of the doping's addition.Additionally, during the drying and fire activities, the behavior of shrinkage of produced brick composites changed according to the kind and concentration of dopants.therefore, these dopants level within the composite's surfaces revealed their excellent compatibility and strong interfacial adhesion between clay and rGO.Moreover, the thermal conductivity, thermal diffusivity, and specific heat capacity were improved gradually depend on the rGO sheets concentration ratio to achieve optimum values of 0.43 W/mK, 0.46 mm 2 /S, and 1.94 MJ/m 3 K at 4%-rGO, respectively.These thermal characteristic variations are prompted by the significant interaction between clay-H 2 O products and oxygen functional groups (epoxides, carboxyl's, and hydroxyls) of rGO nanosheets.Therefore, these interactions lead to a uniform dispersion of the nanosheets within the clay-rGO composite.

Figure 4 .
Figure 4. (a -e) SEM micrographs and statistical histograms of pore size distribution of untreated and treated nanocomposite clay using various ratios of rGO NPs.

Figure 6 .
Figure 6.Drying and firing shrinkage of clay-rGO bricks at different levels of rGO nanosheets.

Table 1 .
Mineralogy of the raw material.

Table 2 .
Thermophysical characteristics of nanocomposite clay bricks with various levels of rGO dopant.